The invention relates to a steering system for a vehicle in which no mechanical connection exists between the steering wheel and the steering linkage of the vehicle.
Such steering systems are known as xe2x80x9csteer-by-wirexe2x80x9d systems. In such systems the steering movement of the steering wheel is transmitted electronically from the steering wheel to the steering linkage by the setpoint steering angle input by the driver being transmitted xe2x80x9cby wirexe2x80x9d firstly to an electronic control circuit which then sends xe2x80x9cby wirexe2x80x9d a positioning command to an actuator which may be embodied as an hydraulic or electric servomotor and produces the steering movement at the front axle steering linkage. In this arrangement the electronic control circuit processes signals received from various sensors. The two most important signals are the setpoint value for the steering angle as input by the driver at the steering wheel, and the actual value of the position of the steering linkage at the front wheels resulting from the positioning movement of the actuator. Further signals may be, for example, the speed and the yaw rate of the vehicle.
In addition to the function of dictating the steering movement of the front wheels such a steering system has the task of furnishing the driver with a haptic signal, i.e. giving him a xe2x80x9cfeelxe2x80x9d for the road contact of the front wheels. Thus, a steer-by-wire system not only needs to be able to translate the steering movements of the driver into positioning movements at the wheels but also to produce the restoring forces at the steering wheel relating to the cornering forces transmitted between the road surface and the front wheels. For this purpose a further actuator is needed which is connected to the steering wheel, this actuator too, being signaled electronically. However, it is not the function of this actuator to implement a positioning movement, it instead serving to produce a haptic counter moment at the steering wheel for the driver. For this purpose the electronic control circuit needs to process information as to the cornering forces at the wheels of the front axle. This information is made available either by force sensors arranged in the steering linkage or is derived from the hydraulic pressure acting on the actuator or the applied current strength to indirectly receive a measure for the cornering forces at the wheels of the front axle.
The advantage of such a steering system as compared to a conventional, mechanical steering system is, more particularly, that the steering translation is freely selectable, for example, as a function of the speed of the vehicle. The boost in the steering force is likewise freely programmable. As an additional function an active, dynamic correction of the steering angle may be provided serving to enhance the steadiness of driving. This function is comparable to that of so-called ESP systems which provide automatic braking in enhancing the steadiness of driving. Finally, the steering may also be made use of for automatic tracking and in preventing collision.
One important consideration as regards steer-by-wire systems is their fail-safe response, i.e. single defects in the system, which cannot be excluded with a probability bordering on certainty, must never result in total failure of the system and thus to loss of vehicle steerability. To satisfy this requirement all active safety-relevant chassis systems make use of a fallback based on conventional engineering, enabling a defective active system to be rendered safe by taking it out of circuit. Such systems are, therefore, termed fail-safe. As an alternative multiple redundancy solutions could be provided, in which faults in the redundant sub-systems are automatically compensatable, such systems being termed error tolerant.
The invention provides a steering system with which the functions as cited above are achievable by particularly simple means whilst simultaneously assuring satisfaction of all safety requirements. This is by means of a steering system which comprises a steering linkage, a control hydraulic motor and a control hydraulic circuit, a steering wheel connected to the control hydraulic motor, and a control hydraulic cylinder connected to the control hydraulic motor by the control hydraulic circuit. The vehicle steering system further comprises a servohydraulic cylinder, a servopump, a servohydraulic circuit connecting the servopump to the servohydraulic cylinder, and an electronic control circuit which controls the servohydraulic circuit. The control hydraulic circuit, which is permanently closed, transfers the steering movements of the steering wheel by displacement of a volume of hydraulic fluid non positively to the front wheels. The control hydraulic circuit thus serves as a fallback, i.e. should the system develop a fault, e.g. due to a power failure, the steerability of the vehicle is still assured.
In accordance with one preferred embodiment of the invention it is provided for that a position sensor is arranged at the steering linkage and that an angle sensor is arranged at the steering wheel. The information furnished by the two sensors serves, for one thing, to control the servohydraulic circuit in normal operation and, for another, the information is made use of in emergency operation to operate the servohydraulic circuit as a simple closed positioning loop.
In accordance with the preferred embodiment a modulator pump is provided connected in parallel to the control hydraulic cylinder in the control hydraulic circuit, this modulator pump being capable of supplying a hydraulic flow superimposing that furnished by the control hydraulic motor. The modulator hydraulic circuit thus formed enables a variable steering translation and an active steering angle correction to be made available without the control hydraulic circuit needing to be opened. Depending on the hydraulic flow made available by the modulator pump, the hydraulic flow of the control hydraulic motor can be boosted, diminished or even overcompensated. In conjunction with the position sensor and the angle sensor which sense the position of the steering wheel and the position of the steering linkage, the modulator hydraulic circuit is able to synchronize the position of the steering wheel relative to the position of the steering linkage. This is why an internal leakage in the control hydraulic circuit is tolerable.
Preferably a solenoid valve is disposed between the modulator pump and the control hydraulic circuit, by means of which valve the modulator pump can be decoupled from the control hydraulic circuit. In this arrangement the solenoid valve is urged by a spring mechanically into a position in which the modulator pump is decoupled from the control hydraulic circuit. This configuration too, serves to achieve a high fail-safe response; by mechanically acting upon the solenoid valve, the system automatically returns to fallback when, for instance, no further supply voltage at all is available.
The electronic control circuit is able to receive a vehicle speed signal and a vehicle yaw rate signal and is able to control the modulator pump as a function of these signals as well as of the signals of the position sensor at the steering linkage and at the steering wheel. It is in this way that active dynamic correction of the steering angle is made possible irrespective of the steering movements by the driver.
In accordance with the preferred embodiment of the invention it is provided for that in the control hydraulic circuit a valve is arranged by means of which the connection of the control hydraulic motor to the control hydraulic circuit can be restricted or open-circuited. Thus, it can be prevented that the hydraulic flow furnished by the modulator pump has a reaction onto the control hydraulic motor and thus onto the steering wheel.
Preferably it is provided for that the valve by means of which the connection of the control hydraulic motor to the control hydraulic circuit can be restricted or open-circuited is mechanically urged into a position in which the connection of the control hydraulic motor to the control hydraulic circuit is totally open, this too, serving to enhance the fail-safe response.
Preferably a supply conduit is provided supplying via two check valves the control hydraulic circuit with hydraulic fluid so that even if a small leakage exists in the control hydraulic circuit the functioning of the control hydraulic circuit continues to be assured.
Preferably each of the two check valves is provided with a pilot conduit which is pressurized by the pressure in the opposite line of the control hydraulic circuit. This enables the check valve, in the line of the circuit which is the low-pressure line at the time, to open when a predetermined pressure in the high-pressure line is exceeded, thus preventing an excessively high pressure arising in the control hydraulic circuit.
In accordance with the preferred embodiment of the invention in the control hydraulic circuit two pressure sensors are arranged in the two feed conduits to the control hydraulic cylinder, the signals of the two pressure sensors being made available to the electronic control circuit. The pressure sensors enable the effective steering forces to be sensed and the servohydraulic circuit to be suitably controlled as a function of these steering forces.
Preferably in the servo hydraulic circuit two pressure sensors are arranged in the two feed conduits to the servohydraulic cylinder, the signals of the two pressure sensors being made available to the electronic control circuit. These two pressure sensors serve to feed back the actual value of the boosted steering force to the electronic control circuit.
Advantageous aspects of the invention read from the sub-claims.